Commercial processes for preparing polyalkylene terephthalates by direct esterification of terephthalic acid (TPA) with linear glycols, such as ethylene glycol (2G), 1,3-propanediol (3G) and 1,4-butanediol (4G), are well known in the art. Continuous processes for preparing polyethylene terephthalate (2GT) at commercially acceptable throughputs can be carried out to high TPA conversion without the use of an esterification catalyst by performing the reaction at sufficiently high temperatures, typically greater than 270° C. Increasing the reaction temperature typically acts to accelerate the reaction of TPA and thereby improve throughput. Unfortunately, 3G and polytrimethylene terephthalate (3GT) are thermally unstable at high temperatures. Operation of a continuous process for preparing 3GT at such temperatures would generate harmful byproducts, such as acrolein and allyl alcohol, and lead to poor polymer quality.
TPA-based routes to 3GT polymer suffer disadvantages relative to routes based on use of terephthalate esters due to the low solubility of TPA in 3G, which retards its reaction, and, in a continuous process, insufficient reaction of TPA results in carryover of unreacted TPA into downstream equipment. The presence of unreacted TPA in downstream equipment can cause operability problems such as TPA deposits creating flow problems. The presence of TPA in finished polymer results in processing problems, such as poor filterability and spinning problems.
U.S. Pat. No. 4,680,376 discloses a process for continuous production of high molecular weight polybutyleneterephthalate by direct esterification of TPA and 4G in the presence of tin- or titanium-containing catalysts.
U.S. Pat. Nos. 6,277,947 and 6,326,456 disclose a process for preparing 3GT by esterification of TPA with trimethylene glycol in the presence of a catalytic titanium compound, precondensation and polycondensation. The esterification is effected in at least 2 stages.
However, there is no commercial process for continuously producing 3GT using TPA. It is highly desirable to develop a contiguous process, especially a low temperature process for esterifying TPA with 3G, that can minimize the generation of harmful byproducts disclosed above and provide TPA reaction rates to maintain commercially attractive throughputs. It is also highly desirable to develop a contiguous process that uses low quantities of catalyst and is capable of operating at total residence times in the esterifier of less than 4 hours to minimize discoloration and improve polymer quality. It is further desirable to develop a process for producing 3GT by a continuous esterification of TPA with 3G to produce an oligomer and using the oligomer for polymerization to high molecular weight 3GT polymer that does not exhibit filterability problems during spinning.